如何使用SQLAlchemy核心来选择除postgresql中的1个特定列以外的所有表列？

Question

I use mostly SQLAlchemy core(v.1.0.8) expression language with flask(0.12) to create API calls. For a particular case where the table has 20 columns, I wish to select all except 1 particular column. How can this be done in the 'select' clause? Is there anything like 'except' that can be used instead of explicitly selecting the columns by names?

Answer 1

Normally you'd explicitly name all wanted columns, omitting the column not wanted in the dataset.

stmt = sa.select([MyTable.col1, MyTable.col2, ...])

Notice that the select object accepts a list. So, here it is possible to use list comprehension with an if condition to get the same effect as except :

filt_out = 'col_name_not_wanted'

stmt = sa.select([col for col in MyTable.__table__.columns if col.key != filt_out])

The filter condition can even be another function, and thus you could have more complicated logic.

Example:

def filt(col):
    if col.index:
        return col.primary_key is True
    elif 'secret' in col.key:
        return False
    else:
        return True

stmt = sa.select([col for col in MyTable.__table__.columns if filt(col)])

The above snippet assumes MyTable is a sqlalchemy Declarative Mapping. If instead you have sqlalchemy Table, replace MyTable.__table__ with Mytable

Here's a complete example:

import sqlalchemy as sa
import sqlalchemy.ext.declarative as dec

b = dec.declarative_base()

class M(b):
    __tablename__ = 'm'
    id = sa.Column(sa.Integer, primary_key=True)
    col1 = sa.Column(sa.Integer)

stmt = sa.select([col for col in M.__table__.columns if col.key != 'col1'])
print(stmt)
# prints:
SELECT m.id 
FROM m

Answer 2

您可以进行一些聪明的回顾，但是为什么不选择全部而忽略不需要的呢？